A. Field of the Invention
The present invention relates to electrostatic devices used to align particles unidirectionally in particleboard manufacture. More particularly, the present invention is directed to an improved electrostatic plate construction which ensures that the electric field lines between adjacent charged plates will remain substantially horizontal and undistorted adjacent to the surface of a particleboard mat being formed.
Electrostatic orientation units generally comprise a plurality of upright electrically charged plates arranged over a traveling conveying system. In order to produce fiberboard or particleboard, fibers or particles are dispersed from a hopper or the like so that they pass downwardly between the charged plates. The fibers or particles are then aligned between the plates, due to electrostatic forces, and fall to the conveying system so that a mat of fiberboard or particleboard is produced. However, several problems exist with known orientation units. For instance, assuming an orientation unit which utilizes DC current for positively and negatively charging alternate plates, it can be appreciated that the electric field existing between adjacent plates becomes distorted adjacent the mat being formed. Because the electrostatically charged plates must have their bottom edges disposed higher than the mat being formed, particles which have fallen between such plates will not experience the aligning electrostatic force in the area between the top of the mat and the bottom edges of the electrostatic plates. Consequently, a good portion of the particles may not be unidirectionally aligned in the formed particleboard.
It is important that unidirectional alignment occur because random alignment does not provide the strength and stability in the longitudinal direction which is important for particleboard used in roof sheathing, flooring, siding, shelving, plywood cores, etc.
Because of substantial waste in the lumber products industry, it is desirable to provide an orientation unit functional to align relatively large particles. For instance, in the production of plywood, of every 100 pounds of logs, only approximately 40% actually becomes finished plywood. The remaining bark, chips and core may be totally wasted. Bark and wood chips may, however, be effectively used to produce particleboard. Additionally, other materials such as rice hulls, straw, etc., if properly aligned unidirectionally, may be readily used to produce particleboard having substantial strength and stability for use in a variety of applications.
B. Description of the Prior Art
Prior art orientation units have recognized that an electric field adjacent the bottom portion of charged alignment plates may be distorted thereby resulting in misalignment of particles or fibers. For instance, in U.S. Pat. No. 3,843,756, there is disclosed a method for forming boards from particles in which AC current is used to align the particles. Alternate electrode plates are arranged in a series and are charged with opposite polarity from an AC high voltage source. Furthermore, there are secondary electrode plates arranged below the traveling mat which is formed. The secondary electrode plates are used to shape the electric field so that such field is more nearly horizontal in the space just below the bottom of the electrode plates where the mat is being formed. However, it may be readily appreciated that the bottom edges of the primary electrode plates must be arranged in an increasing step construction in the direction of the travel of the mat so that the formed mat will not be gouged or disturbed by the bottom edge of the plates.
DC orientation units have been proposed in which separate groups of aligned orientation plates are staggered one from another in order to compensate for the increasing slope of a mat being formed. Such a construction is somewhat impractical from a cost and construction standpoint.